1. Field of the Invention
The present invention relates to a position measuring system with a scale graduation and with at least one scanning element which can be moved in relation to it, by means of which position-dependent, intensity-modulated output signals can be generated in case of a relative movement, which are fed to a downstream-connected evaluation unit.
2. Discussion of Related Art
The subsequent explanation of the problems whose solution is the basis of the instant invention is provided by means of an example of a magnetic position measuring system. However, in principle similar problems arise in connection with position measuring systems based on other physical scanning principles, for example in connection with optical, inductive or capacitive position measuring systems.
In the case of a magnetic position measuring system, several scanning elements, which are sensitive to magnetic fields, are used for scanning a magnetic scale graduation. Magneto-resistive elements as well as Hall elements, for example, are suitable for this. In the case of an incremental measuring system, a periodic magnetization of a scale graduation, which is movably disposed in relation to the respective elements, is scanned by means of these elements. Here, the periodic magnetization consists, for example, of alternatingly arranged magnetic north and south poles. In the course of the relative movement between the scale graduation and the scanning elements, the outputs of the scanning elements provide periodically modulated signals, which can be further processed in a known manner. In incremental measuring systems, at least one pair of phase-shifted output signals S.sub.1, S.sub.2 is generated as a rule, wherein the phase shift is 90.degree.. A series of requirements now results in regard to the output signals S.sub.1, S.sub.2, if they are intended to be further processed in known evaluation units. For example, the evaluation unit as a rule presupposes sinusoidal output signals, since a dependable further division of the signals or their interpolation is only possible on the basis of the ideal sine shape. Furthermore, within a signal period, the two signals S.sub.1, S.sub.2, phase-shifted by 90.degree., should have amplitudes which are the same as much as possible. In case of a representation of the signal progression with the aid of a Lissajous figure this means that, if possible, a circle should result as the Lissajous figure. It should be stated that a further demand made on the signal quality is that the maximum amplitudes of the output signals S.sub.1, S.sub.2 provided should be subject to minimal possible fluctuations, even if, for example, the scanning gap between the magnetic scale graduation and the scanning element used varies.
Problems resulting in respect to the signal quality in connection with known magnetic signal systems will be explained hereinafter by means of FIGS. 1a, 1b, 2a and 2b. FIG. 1a shows the dynamic characteristic of a scanning element which is sensitive to a magnetic field, for example a magneto-resistive element consisting of a center parabolic area and a saturation area which adjoins the parabolic area. In this case, the relative resistance change in comparison to the detected field strength is plotted here, i.e. the field strength to be detected is used as the input signal, and a defined relative resistance change is the result as the output signal. Furthermore, three modulation areas or operating areas a, b and c of the dynamic characteristic of the scanning element have been drawn in the representation, which are to be compared with each other. These modulation areas a, b and c correspond to different maximum magnetic field strengths or respective input signals, between which the scanning element is operated, i.e. the different modulation areas a, b and c represent different scanning gaps between the scanning element and the magnetic scale graduation. The area a with the least maximum field strengths represents the greatest scanning gap, while the area c represents the least scanning gap. In this case, all selected modulation areas are located in the parabolic portion of the dynamic characteristic of the scanning element or respective several scanning elements.
The resultant Lissajous representation is shown in FIG. 1b, which results for two sine- and cosine-shaped signals S.sub.1, S.sub.2, which are phase shifted by 90.degree., in the different distance areas a, b and c in the parabolic portion of the dynamic characteristic. Here, for one it can be clearly seen that it is assured in the parabolic area of the dynamic characteristic of the scanning element that circular Lissajous figures result. The signal amplitudes of the two phase-shifted signals S.sub.1, S.sub.2 are therefore respectively the same. On the other hand, it becomes clear from the representation of FIG. 1b, that the maximum signal amplitudes in the distance areas a and c differ by almost a factor of 2, i.e. the radius of the two circles nearly differs by a factor of 2. However, a fluctuation of the signal amplitudes of this size at varying scanning gaps causes problems during further processing of the signals in the evaluation unit. Accordingly, based on the extreme distance dependency of the field strength of the scanned scale graduation on the scanning gap, a considerable impairment of the signal quality results.
A further problem in connection with known magnetic measuring systems will be explained with the aid of FIGS. 2a and 2b. In this case, the basis is the same dynamic characteristic of a scanning element sensitive to a magnetic field as in the previous example. However, now the scanning element is operated in three modulation areas a, b and c of the dynamic characteristic of the scanning element, all of which are no longer located in the area of the parabolic portion of the dynamic characteristic of the scanning element, but instead already partially in the saturation area of the dynamic characteristic of the scanning element. Therefore, the maximum input signals or respectively maximum field strengths are respectively located in the saturation area of the dynamic characteristic of the scanning element. In this case, the saturation area should be understood to be the area of the dynamic characteristic of the scanning element wherein, even with significant changes in the detected magnetic field strength no changes in the amplitude of the output signals result, or respectively, the plotted relative resistance change remains approximately constant. The area a again represents the largest scanning gap, while the area c represents the smallest scanning gap with the highest detected maximum field strengths.
As can be clearly seen from the associated FIG. 2b with the Lissajous representation of the phase-shifted output signals S.sub.1, S.sub.2, a circular Lissajous figure still results in the area a with the greatest scanning gap, i.e. equal signal amplitudes of the two phase-shifted output signals result. However, the smaller the scanning gap is selected, i.e. the more the scanning element is modulated in the saturation area, the greater the deviation of the Lissajous figure from the ideal circular shape becomes. This is the case in particular in the area c, where a clear distortion of the formerly circular Lissajous figure can be seen.
Such a dependence of the shape of the Lissajous figure on the scanning gap also causes undesirable errors in the further processing of the signals in the evaluation unit, for example during signal interpolation.
As already implied above, similar problems also arise in connection with position measuring systems based on different physical scanning principles.